Repeatered transmission system



April 28, 1942.

E. L-GREEN REPEATERED TRANSMISSIN SYSTEM Filed May 31, 1941 3Sheets-Sheet 1 April 28, 1942. 1.1 GREEN 2,281,035

REPEATERED TRANSMISSION SYSTEM` Filed May 3l, 1941 3 Sheets-Sheet 2 /NI/E N TOR ATTORNEY April 28, 1942.

Filed May 31, 1941 Sheets-Sheet 3 ql ["n Pr Pr 9/ fr i, FIG-3 lscr Rrcr6/ aff 2 2 ../6 VVVAV VVV 23 l 76) 8) a 1 L, 3 /03 Moo HPF Amo LPr D l-7l) E mM/s. (3L TEMI. 'l'

.von Pr ,van pr /N VEN TOR E l. GREEN BV ATTORNEY Patented Apr. 2.8,1942 UNITED STATE REPEATERED TRANSMISSION SYSTEM Estin I. Green, Multum,N. J., assignmto Ben Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application May 31, 1941, Serial No.396,148

rc1. 17a-44) 10 Claims.

This invention relates to long distance elec# tric signaling systems inwhich duplicate transmission facilities are provided to insurecontinuity of service.

It has been proposed heretofore to separate along distance signalingsystem having duplicate transmission facilities into a multiplicity ofsections or links so that if in any link the regular or workingtransmission facility becomes impaired, it may be replacedby the spareor idle facility in that link. Where provision is made for effecting thesubstitution automatically under the control of currents transmittedwith the signals, asin the wire line system disclosed in I. G. WilsonPatent No. 2,229,158, January 21, 1941, for example, it is found thatunder some conditions encountered in practice there is a possibilitythat unless special precautions are taken the occurrence of a necessarysubstitution in one link may give rise to unnecessary substitutions inother links or that the automatic replacement of an impaired facility inone link may occasion repeated linterchange of the two facilities in adistant link. y

A principal object of the present invention is to provide an improvedsystem of the kind described and more particularly to eliminateunnecessary or false operation or to reduce substantially the likelihoodof its occurrence.

The embodiments of this phase of the present invention that arehereinafter described in detail are specifically adapted for a wire linetransmission system of a type such as the coaxial system employing twoone-way transmission lines each of which is provided with signalrepeaters having automatic gain regulation under the control of a pilotwave transmitted with the signals. The system is divided into aplurality of links for switching purposes and it is so arranged that atthe input or sending end of each link both lines receive the signals andpilot waves from whichever of the two lines in the preceding linkhappens to be the working line. At the output or receiving end of eachlink automatic switching means are provided which operate in the eventthat transmission over the regular line is impaired to connect the idleline in that link to the input end of the next linkY and to disconnectthe regular or working line therefrom. Transmission, in a given link isdeemed to be impaired if the pilot wave received at the output end ofthe working line fails or if otherwise the intensity of the receivedpilot departs .from a preassigned normal value by more than a certainamount. In response to such failure or abnormal departure of the pilot,the switching mechanism is automatically' set into operation.

Gain regulated repeaters at the receiving end of each switching linknormally operate in the two lines to maintain the pilot output intensityin each line approximately at a predetermined constant value, which isthe same for both the working line and the spare line. If the twointensities are exactly alike and there is then an abrupt failure of thepilot on the working line, quickly followed by substitution of the idleline, the repeaters at all subsequent switching points experience only amomentary effect inasmuch as the pilot received by Way of thesubstituted line is of the same intensity. as the pilot previouslyreceived by way of the regular line.

Under some conditions, however, the pilot may not fail abruptly at agiven switching point but its intensity may gradually drift in onedirection or the other. beyond prescribed normal limits. Substitution ofthe idle line will take place in the affected link as before when theprescribed limit is passed but with this this difference. pilot driftsfrom its normal value, to a lower value, for example, the gain regulatedrepeaters in the following section of Working line will continuallyaccommodate themselves to the changing pilot intensity thus maintaining,at the next 'switching station a constant pilot output. When thedrifting pilot passes through the prescribed limiting value and thesubstitution of lines is made in the affected link, there appears atthat next switching point a sudden increase in the pilot intensity equalto the vdifference in the intensities of the two pilots at the receivingend of the affected link. This sudden increase in pilot intensity iscommunicated almost instantly to all subsequent switching points,inasmuch as the various regulators do not operate instantaneously, andif at any such subsequent point the pilot output intensity happens to beat the moment of the switching somewhat higher` than the prescribednormal value although within the prescribed limits, the increase inpilot intensity incidental to switching will raise the total pilotintensity at that point above the upper normal limit and cause a switchto occur. This switch is obviously unnecessary for in a few moments theregulators would have reduced the pilot intensity to a value within thenormal range and transmission would have been quite normal.

So also if the pilot output intensityv in the idle line of the affectedlink happenedto be below its normal value at the moment of switching',the comparatively large change in pilot intensity As the switchingpoints.

If the circuitfis so arranged that a manual adjustment is required aftera switch occurs before` the switching mechanism is in condition to makeanother switch, only one false switch can ,take place at any one point.The circuit, however, may be fully automatic so that it will switch backand forth between regular and spare lines whenever the pilot level onthe working line departs from'its normal range. In such a fullyautomatic system it is theoretically possible to have as many successivefalse switches at a distant switching point as there are switchingpoints between'it and the initial switching point.

In general, false switching may occur under either of two circumstances,(1) when there is a drift of vthe pilot level on the working line beyondthe normal limits while the pilot level or. the idle line remains withinthe normal range, and (2) whenthere has been a drift of the pilot levelon the idle line in ,one link, the level on the working line in afollowing link having varied from normal in the same direction, and theworking line of the rst-mentioned link, being normal, becoming subjectto a sudden failure.

In accordance with this phase of the present invention as embodied intwo systems illustrated in the drawings, the objective of reducing oreliminating false switching is attained by automati-l cally adjustingthe relative pilot levels on the two lines at each switching point sothat so long as they remain Within `the prescribed normal y the I. G.Wilson patent supra, such a system may provide for a phase displacementof 120 degrees between the currents transmitted through the twofacilities and a vector addition of the two currents at a receivingpoint. If the two vectors so combined are of equal amplitude, thevectorresultant at the receiving point will be of the same amplitude as eithercomponent alone, hence 7,

if one facility is suddenly disabled .there will be little or no changein the amplitude of the received current and service can continuesubstantially unimpaired.

' Another object of the present invention is to obvate the falseoperation of marginally operative devices in a system of the kind lastdescribed that might occur if the two current vectors were not exactlyequal. Even though the two facilities be provided with automatictransmission regulators these would ordinarily permit a limited range ofrelative amplitude variation such that the vector resultant at a givenpoint might be sensibly different than either vector alone. Moreparticularly the difference, in respect of the amplitude of pilot orcontrol currents transmitted through the system, might be sufcient thatin the event of sudden failure of one facility, various marginallyoperative devices responsive to the control current would be subject tofalse operation. These devicesmay be for example alarm circuits andswitching devices adapted to disconnect impaired facilities.

In accordance with a feature of the present invention provision is madefor accurate equalization of the transmission levels in the twofacilities at points where vector combination of the respectivetransmitted cur'. ents is effected. In accordance with another feature,the duplicate facilities are divided into a multiplicity of links withvector combination of the currents at the junctions' thereof andpreservation of the 120-degree relation at the input end of each link.

The nature of the present invention and its various features, objectsand advantages will appear more fully from a consideration of thefollowing description of the embodiments illustrated in the accompanyingdrawings. In the drawlngs: i

Fig. 1 illustrates an embodiment of the invention comprising duplicate,alternatively operative Atransmission lines in which the pilot output ofthe working line varies the pilot output on the idle line in the samedirection and degree;

.-Fig. 2 illustrates a modification of Fig. l in which the pilot outputsof the two lines are at each switching point continuously andautomatically adjusted to equality;

Fig. 3 illustrates an embodiment of the invention comprising duplicate,simultaneously operative transmission lines in which the transmissionlevels on the two lines are accurately equalized in the manner of Fig.2: and

Fig. 4 further illustrates the Fig. 3 system.

Referring more particularly now to Fig. 1, there is illustrated a systemof the kind described com prising two transmission 1ines,` LI and L2,arranged for transmission in the same direction betweena pair ofterminal stations not shown. For switching purposesthe line system isdivided into a multiplicity of tandem links with a hybrid coil at eachjunction adapted to receive signals from one or the other of the linesin the preceding link and to apply them to the transmit ends of both ofthe lines in the following link. At the output or receiving end of eachlink there are provided switching means which. operate under the controlof pilot currents transmitted through the system along with the signalsto connect one or the other of the lines throughv the hybrid coil to theinput end of the next link. Thus in each link both lines are normallycontinuously ener` gized with signal and pilot throughout their lengthsbut only one is effectively connected at its output end for throughtransmission.

Repeaters 2 are interposed at intervals in each line and there may be aplurality of them in each link in addition to those at the junction orswitching points, viz., repeater 3 in line LI and repeater 23 in lineL2. All of these repeaters are of a type adapted for automatic gainregulation under the control of pilot current transmitted with thesignal, and they normally operate toy maintain the transmission levelsat their respective outputs within xed limits despite fluctuations inthe attenuation of the transmission line and other effects tending tochange the transmission equivalent of the system. The pilot currentsthat con trol the switching means may serve also for the `amplifier forthe diverted pilot current and 'a repeater gain control mechanism -5that is autoi of pilot output intensity, and the gain control mechanismcomprises a silver sulphide element or other thermistor. connected ingain-controlling relation in the repeater-amplifier circuit.

The switching mechanism includes a relay 1 Y that is associated with theoutput end of line LI and a corresponding relay 21 associated similarlywith line L2. These relays may, if desired, be of a type consisting oftwo magnetic reeds surrounded by a glass envelope containing a sultablegas such as helium, the contact between the reeds being actuated by anexternal magnetic field, all as disclosed in the pending application ofL. R. Schreiner, Serial No. 342,635, iiled June 27, 1940. Thisconstruction permits the outer conductor of a coaxial circuit to becarried continuously through the relay. In one condition of the system,neither of the relays 1 and 21 is operated, in which case line LI isconnected b'y relay 1 to the input terminal of the hybrid coil 5I in thenext section and line L2 is connected by relay 21 to a terminatingresistor 28. `In an'alternative condition of the system both of theserelays are operated and in this case line LI is disconnected from hybridcoil 5l and terminated i by resistor 8 while line L2 is disconnectedfrom terminating resistor 28 and connected to the input Iterminals ofhybrid coil 5I.

-As previously intimated, the system may be arranged for semiautomaticoperation, that is,

for a single automatic transfer from the' regular the latter becomesimpaired, and, without the intervention of an attendant, a switch may bemade in the reverse direction if the substituted line then becomesimpaired and the other line has in the meantime been restored to normal.First to be described is the semiautomatic arrangement. This requiresthat the resetting or restore key Il be closed and that a circuitshunting the key be brok'en at the point a-a.

Treating line LI as the regular line and line L2 as the spare, Vandassuming that the circuit is arranged for semiautomatic operation.relays 1 and 21 are then normally in the unoperated condition and lineLI is connected for through transmission. To effect a switch to line L2in the event of impairment of line Ll, means are provided. responsive toan abnormal change in the pilot level at the output of repeater 3, tooperate relays 1 and 21. The operating means comprises a detector 9 thatis connected to receive the diverted pilot from the output of repeater 3and that is so constructed, as by incorporating a pair of differentlybiased relays. that if the intensity of the pilot applied to it departsfrom a preassigned range a relay l0 in its output circuit is immediatelyoperated. If'as suggested the pilot intensity at the output of repeater3 is normally held by the gain regulating system to a range of plus orminus three decibels, the operating margin of detector 5 can be arrangedto coincide,

orit may be otherwise so arranged as to operate when predeterminedlimits are exceeded.

Associated with the' output of repeater 23-and controlled by pilotcurrent diverted therefrom is a detector 29 and a relay 30 whichrespectively correspond with detector 9 and relay l. As long as thepilot intensity at the output o repeater 23 is within the normal range,or in other words as long as line L2 is unimpaired and in condition tobeused for replacing line Lil, relay Mi is unn operated and its upperbreak contact is closed. Then on operation of relay l@ in response to afault on line Ll, an operating battery circuit for relays 1 and 21 isclosed through the upper and lower make contacts of relay i@ in seriesand the upper break contact of relay 33t). Thereupon relays 1 and `21operate and line L2 is thereby substituted for line L! for throughtransmission. At the same time an operating battery circuit for alock-up relay d2 is closed through the upper make contact of relay l@and the upper break contact of relay 39. The consequent operation ofrelay t2 closes a holding circuit through one of its make contacts andkey di. With relay t2 in its locked position, a holding circuit forrelays 1 and 21 is completed through one of its make contacts. A lamp 44in series with the operating winding of relay 42 lights Whenever thisrelay is in its operatedcondition, or in'v other words as long as thespare line is in use.l

It will be noted that on manually opening restore key 4|, the'operatingcircuit of lock-up relay 42 will be broken, andif line LI is unim pairedat the time so that relay I0 is in its unergized, thereby released, andcause line Ll to be 'substituted for line L2.

For fully automatic operation of the switching system key 4I is heldopen, or removed, and the shunting circuit is completed through a-a. Thelower break contact of relay 30 now serves ir. lieu of the key Il. Innormal operation over line LI, the last-mentioned break contact isclosed, sothat when occasion arises a switch can be made to line L2. Ifon that occasion line L2 is impaired, and relay 30 is therefore in itsoperated condition, both contacts of that relay are open and a switch isprecluded.

After a switch is made to line L2, lock-up relay 42 holds switchingrelays 1 and 21 in their operated condition even though line Li isrestored to normal and relay I0 is thereby released. If now line L2becomes impaired and relay 30 therefore operates, the opening of thelower contact of relay 30 breaks the operating circuit of lock-uprelay42 thereby releasing relays 1 and 21 and restoring the system to. itsoriginal condition, that is, lwith line LI in service. Thelast-describedswitch is precluded, however, if line Ll is still impaired and relay l0is therefore still operated.

The Fig. `1 switching arrangement as above described, whetherconditioned for semiautomatic or fully automatic operation, is such thata normal or necessary interchange of lines in one link may give rise tofalse switching in other links under certain conditions hereinbeforedescribed in detail. These conditions, it will be recalled, involve thefact that whereas the various pilot levels in both lines at allswitching points may lie within the normal range just prior to theoccurrence of a fault, these levels may have drifted from the center ormean value of their respective ranges in such relative directions thatthe sudden change in level ordinarily to be expected on substituting oneline for another may be enough that the normal range is momentarilyexceeded at various switching points. In accordance with a feature ofthe invention provision is made in Fig. 1 to reduce the likelihood ofsuch false switches occurring.

More particularly, provision is made in Fig. 1v

for varying the transmission equivalent oi' the idle line in synchronismwith the variations.v in the pilot output level on the working line sothat at the instant before switching the pilot output levels on bothidle and Working lines are at least approximately equalized. Forthisfpurpose an auxiliary amplifier 6 of .adjustable gain is interposedin line LI following repeater 3 and a similar auxiliary amplifier 26 islikewise disposed at the output of repeater 23. The gain adjustingmechanism of each auxiliary amplifier is illustrated as comprising anindirectly heated thermistor, viz., thermistor element I I and heater I2for amplifier 6 and thermistor element 3| and heater 32'for amplifier26.

Heating current for 'each of the thermistors is derived from a -constantcurrent source when the line with which it is associated is the workingline, and from the pilot current on the other line when the line withwhich it is associated is the idle line. Thus a rectifier I3 connectedin parallel with the input of detector 3 supplies amplified, rectifiedpilot current for .heater 32 through a break contact on relay 42 that isclosed as long as line LI is the working line, while heater I2 issupplied through another break contact on relay 42 with constantcurrentfrom a source 45. When relay 42 is operated, incidental to substitutionof line L2 for line LI, heater 32 is supplied with current from aconstant current source 'through a make contact of relay42 and heater I2is supplied with current through another Amake conthe reference value,zero decibels, when the in-V tensity of the pilot in the opposite lineis exactly at its mean value, that is at the center of its normaloperating range. The circuit parameters can be and are further soselected that any change in the intensity of the heating current derivedfrom the pilot in one line givesrise to an equal change in the samesense in the gain of the auxiliary amplifier in the other line.

If for example, then, line LI is the working line and the pilotintensities at the output of-repeaters 3 and 23l arel at theirrespective mean values, the gain of auxiliary amplifier 6 is held atzero value by the heating current supplied from source 45 and theauxiliary amplifier 26 is likewise maintained vat 'zero gain under thecontrol of the `pilot current derived from line LI. Now if the pilotlevel on line LI beginsvto` change, the gainof auxiliary amplifier 26 inline L2 changes indirect proportion so that the pilot level on line L2at the output of amplifier 26 is maintained exactly equal to that online LI. It will be apparent that if there should be a sudden failure ofline LI under these conditions, there would be only a momen- .tarychange of pilot level experienced at-subsequent switching points andfalse switching would not occur.

' In general it may be said that the Fig. 1 ar-- rangement operates ateach switching point to restrict the differential between the pilotlevel on the two lines to a value not exceeding half the normal range ofeither pilot. In the embodiment of the invention illustrated in Fig. 2,on the otherhand, the differential may be further reduced and in facteliminated so that a switching operation atanygiven switching point Ygives rise to no sensible change inpilot intensity tact on the samerelay from the output of a rec- 4 l.

at .subsequent switching points.

In Fig. 2 the'switching mechanism is essentially the same as that shownin Fig. 1 and corresponding elements are assigned the same refer.encecharacters. A principal difference that will be noted is theshifting of the auxiliary amplifiers 6 and 26 to a position precedingthe point at which pilot currents are diverted for control of theswitching mechanism and the provision of separate pilot filters for theautomatic gain regulators of repeaters 3 and 23. As in Fig. 1 the pilotintensity at the respective outputs of repeaters 3 and 23 is maintainedwithin prede-v termined limits of plus or minus 3 decibels for example;the gain of theauxiliary amplifier in the working line is maintained ata constant reference value of zero decibels, for example; and the` gainof the auxiliary amplifier in the idle line is continuously adjusted inaccordance with the pilot intensity on the working line. In Fig. 2,however, the adjustment of the gain of' the auxiliary amplifier in theidle line proceeds automatically until the lpilot level at its output isequal to the pilot level at the output of the other auxiliary amplifier.The pilot equalizing equipment in Fig. 2 comprises a differential relay41', the opposing windings of which are supplied with operating currentfrom the rectifiers I3 and 33. Relay 41 is connected through contacts onrelay 42 to control the operation of reversible motor 43 which drives arheostat 50. lIfhe adjustment of the latter controls the amount ofbattery current that is supplied to thermistor heater 32 through a breakcontact lon relay 42 when line LI is in use, and it also controls theamount of battery current supplied to the heater I2 through a makecontact on the same relay when line L2 is in use. 'I'he circuit elementsare so proportioned that when the outputs of rectiiiers I3 and 33 areequal, the armature of dierential relay 41 stands in its neutralposition.

Assume that line Lil is the working line and that the pilot intensity at.the output of auxiliary amplifier 6, initially equal to that at theoutput of auxiliary amplifier 26, begins to drift, for example, todecrease. Thereupon the armature of differential relay 41 is moved toits lower contact and motor 49 together with the wiper arm of -rheostat50 are thereby causedto rotate. This changes the heating current to'heater 32 and in such sense as to decrease the gain of auxiliaryamplifier 26.

Operations continue until the gain is so reduced that the intensities ofthe two pilots are equal, whereupon the armature of differential relay4l assumes its neutral position and stops the motor 43. Similarly ifthev pilot intensity on working line'LI relatively increases, thearmature of differential relay 41 is moved to its upper contact andmotor 43 is driven in the opposite direction to raise the sain ofamplifier 26. When line L2-is the working line, the gain of auxiliaryamplifier 6 in line LI is controlled in the same manner. It will benoted, however, that in this case movement of the armaturev ofdiierential relay 41 to either of its contacts requires that motor 49 bedriven and rheostat 50 so that under all conditions the wiper arm ofrheostat D is precluded from leaving its associated resistor.

In Fig. 3 the invention is shown embodied in another form in a systemcomprising duplicate transmission facilities that normally are.bothconnected for concurrent through transmission. In this case it isassumed that the two lines are energized with the same signal and pilotcurrents but displaced in phase relative to each other by' 120 degreesthroughout the frequency range of interest.v At the end of each link thecurrents in one of the lines, say L2, are reversed in phase and thecurrents from the two lines are then applied to respective terminals ofa hybrid coil 6|. These terminals are in conjugate relation with eachother but are in transfer relation with the output terminalsof thehybrid coil, the latter` being connected to Vtransmit the vector sumofthe applied currents to the outgoing line LI. The other line L2 of thenext link receivesl the current of reversed phase from line L2 in thepreceding link through a level adjusting pad 62. Further details of theover-all system will be described with reference to Fig. 4.

As previously indicated the regulated amplifiers 3 and 23 at the end o fthe transmission link shown in Fig. 3 operate to maintain the intensityof the currents in the two lines approximately equal, whereas foroptimum performance of the system and freedom from false operation ofvarious elements therein, the two intensities should be held to a higherdegree of equality. In accordance with the present invention, therelative level equalizing means oi' Fig. 2 is utilized tion from aterminal station at the left through the duplicate transmission lines Lland L2 which are subject to the same changes in attenuation and whichfor specific example may be coaxial conductor pairs within the samecable. The signals'to be transmitted may comprise a multiplex oftelephone channels or television signals, ranging in either case from 64to 2064 kilocycles per second for specific example. Whatever theircharacter, the signals from the signal source 1l are' applied to. thetwolines LI and L2 with equal amplitudes in the two lines but displaced inphase relative to each other by 120 degrees throughout the signalfrequency range.

The phase displacement is effected by applying the signals from source'll to the respective with certain simplifications for this purpose. As

shown in Fig. 3, only one auxiliary amplifier of adjustable gain need beused, just as only one auxiliary amplier is variably controlled in Fig.2 while the other line is in service. If auxiliary amplifier 26 is theone retained as indicated in Fig. 3, its gain is continuallyadjusted bymeansA of reversible motor 49 and rheostat 50 as before so that the gainadjustment proceeds until the current level at the output of amplifier26 is the same as that at the output of repeater amplifier 3. For theequalizing function detectors 9 and 29 need not be used and rectiiiersI3 and 33 may be connected directly to the control windings of relay 4l.The two detectors, however, may be used for other purposes as forexample for sound-` ing an alarm or for disconnecting one of the linesif it becomes impaired. As inFig. 2, the gain control for auxiliaryamplifier 26 may be so adjusted that the gain is zero when the controlpilot level on line LI is at its mean value.

The transmitting terminal station for the Fig. 3 system and thecombining circuits at successive llink junctions are shown in Fig. 4with the level modulators 12 and 'i3 which are supplied with carriercurrent from a source 15. The frequency of the latter may be, forexample, 8 megacycles per second. A phase shifter 'I4 is interposed inthe lead from carrier source 15 to one of the modulators 'i3 and it isdesigned to introduce a carrier phase shift of 120 degrees. Of the twoprincipal second order side-bands produced by a modulators 12 and 13,either the two upper sidebands or the two lower side-bands `are to besuppressed, and it may besupposed, for example, that high-pass filters16 and 'Il are adapted to pass only the upper side-bands. The latter arethen applied to the respective modulators 18 and 19 which arerespectively supplied with carrier currents in like phase from source15. The two lower side-bands resulting from this operation are selectedby respective low-pass filters and 8|, individually amplified andapplied to the respective lines Ll and L2. It can readily bedemonstrated that the two side-bands thus applied to the line areidentical with the original signals from source 'H but displaced inphase from each other by degrees. Pilot currents to be transmittedthrough 4the system may be 'derived from the transmittingterminal 'ilconsignals and pilotsin the two lines can be com- Y bined andthe vectorresultant will be equal in magnitude to either of the component vectorsalone. Thus, if the two vectors are maintained exactly equal inamplitude, as bymeans of the Fig. 3 level equalizer, and one lineshouldfall, transmission would continue through the other without anyabrupt change in amplitude or other effect except a phase shift of 60degrees. The latterphase shift is small enough that carrier telegraphsignals which might be present on the line would be transmittedsatisfactorily.

Once the currents from the two lines are combined, some such means asthat provided at the transmitting terminal station would ordinarily berequired toproduce another set of phasedisplaced currents fortransmission through the next link of the system. In Fig. 4, however,this necessity is avoided and other advantages secured by providing, atthe output end of each link, means for reversing the phase of thesignals-in one of the two lines and means for applying'to the outgoingsection of the other line the vector resultant of the incoming signalsin that line and the phase-reversed signals from the other line.

More specifically, as shown in Fig. 4, the rehybrid. coil is applied toline L2.

. maintaining the intensities of said pilot currents applied directly tothe next section of line L2 through a pad 85 which compensates for theloss occurring in hybrid coil 6I. Brief consideration will disclose thatthe two sets of currents thus delivered to the next section of thetransmission system are again in 1Z0-degree phase relation with respectto each other. The repeater station at the end of the next link is thesame as the f first except that the phase-reversing amplier IBB isdisposed in line LI and the resultant of the vectorial addition of thetwo signals in the Alternate Junctions in the system are alike in therespects described.

` Although the present invention has been described largely in terms ofillustrative embodiments, it will be understood that the invention issusceptible of embodiment in various other 'forms within the spirit andscope of the appended claims.

What is claimed is:

1. A long distance signaling system with duplicate transmissionfacilities comprising two repeatered transmission lines interconnectingthe same points, a plurality of switching stations separating saidsystem into a multiplicity of tandem links each having an input and yanoutput'end, means for applying pilot waves and signals concurrently toboth of said lines at the input end of each of said links, switchingmeans at the.

output end of each of said links for connecting said lines alternativelyto the input end of the next link, said switching means being controlledby the intensity of pilot waves received from` the line that is soconnected to the next link and operative to effect the alternativeconnection when said intensity'departs from anormal limited range,automatic transmission regulating means normally maintaining the pilotintensities in both of said lines at the output end of each link withinsaid limited range, and means constricting the differential between saidlast-mentioned intensities to a value substantially less than saidlimited range. I

2. A long `distance signaling system comprising regular and sparefacilities in a multiplicity of 2,281,035 .peater station at the' end ofthe first link comity of long ydistance transmission links each havingan input end and an output end, means for maintaining pilot current onboth of said facilities in each of said links, transmission regulatingmeans respective to the two facilities normally within a predeterminedrange at the output end of each of said links, means at each of saidlinks responsive to a departure of the regulated pilot intensities fromsaid predetermined range for disconnecting the affected facility, andmeans at each of said links fornormally restricting the differentialbetween said regulated pilot intensities.

4. A long distance transmission system 'comprising duplicatetransmission facilities separated into a multiplicity of links for theconcurrent transmission of the same signals, means for maintaining a1Z0-degree vector relationship between the respective signals applied tothe two facilities, 4means at each junction of said links for reversing.the phase of the signals received from one of the facilities of thepreceding link relative to the phase of the signals received from theother, means for applying the signals of reversed phase to one of thefacilities of the next link, means for vectorially combining saidsignals of reversed phase with said signals from said other facility andmeans for applying the vectorially combined signals to the otherfacility in v the said next link.

5. A combination in accordance with claim 4 in which alternate links ofthe one facility receive said vectorially combined signals and in whichthe intermediate links of said one 'facility receive l said signals ofreversed phase.

6. A signaling system comprising duplicate transmissionfacilitiesseparated into a multiplicity of long distance transmissionlinks, means for applying signals and pilot waves to the twotransmission facilities in each of said links,

vmately constant, and means at the junction of said links operativeunder the control of said pilot waves for equalizing the transmissionlevels in the two facilities irrespective of'variations in relativelevel permitted by said regulating means.

7. A system comprising two long distance transmission lines eachincluding repeaters intersuccessive transmission links, means fortransthe operating margin thereof, and means asso- 4cated with each ofsaid switching means for reducing the intensity differential between thecontrol-'currents in the two facilities.

3. In a signaling system comprising duplicate transmission. facilitiesseparated into a multiplicposed in tandem therein, means fortransmitting control currents through both of said lines in the i thesaid control currents in the two lines,

8. A combination in accordance with claim 7 comprising means at said-onerepeater substanti ally equalizing said intensities independently ofsaid regulating means.

9. In4 combination, two long distance transmission lines extending overthe same route,

.means for transmitting control current through each of said lines,transmission regulating means normally maintaining said control currentwithin a limited range at a plurality of points along said lines, meansat one of said points for automatically replacing the control current inone of said lines with the control current from the other of said linesfor transmission to another of said points, means at said other pointmarginally operative by the control current in one of said lines. andmeans at said one point continually restricting the intensitydifferential of said control current at said one point to a valuesubstantially less than the operating margin of said device.A

10. 'A system comprising a pair" of duplicate long distance signaltransmission facilities separated into a multiplicity of links, eachwith a plurality of amplifying repeaters therein, interlink couplingmeans Asupplying signal and control currents from the output end of eachlink tothe input ends of both facilities of the next following link,transmission regulating means normally maintaining the said controlcurrent in both of said facilities within-a limited intensity range atthe output end of each of said links. means marginally operable by saidregulated control current transmitted through said system, said couplingmeans being adapted to supply signal and control currents as aforesaidfrom the output end of either facility alone in a given link in theevent of failure of the other facility in that link, and /rneansinhibiting false operation of said marginally operable means incidentalto failure of one facility in a preceding link, comprising meanssubstantially equalizing the intensity of the regulated control currentin the two facilities at the output end of each of said links.

ESTILL I. GREEN.

